The incipient motion threshold of microplastics (MPs), defined as the condition that is just sufficient to initiate MP movement, is key to assessing the transport and fate of MPs in water bodies, yet only a few studies have focused on its prediction. This study experimentally investigated the effects of bed roughness (smooth and rough beds) and MP properties (shapes, sizes, and densities) on the incipient velocity (Ui) and critical shear stress (τc) of exposed MPs in an open-channel flow. For a total of 19 types of MPs, Ui and τc were found to range from 0.06 to 0.21 m/s and 0.01 to 0.075 N/m2, respectively. The commonly used thresholds for sediment transport, for example, critical shield parameter Θc and movability number Λc, were established for MPs based on τc. Based on the experimental data of the literature and this study, it was found that predictions of Θc and Λc for sediments do not apply to MPs. A new explicit formula for Λc was proposed for predicting the incipient motion of MPs by introducing the dimensionless particle diameter (d*) and a new dimensionless parameter related to the particle size, density, and shape. The new formula has an absolute error of 12.3%, which is smaller than the existing formula for MPs (55.6%).
Keywords: critical shear stress; incipient motion; incipient velocity; microplastics; shields diagram.